Organic EL display panels include luminescent elements that utilize the electroluminescent properties of certain organic compounds (organic EL elements).
An organic EL display panel is manufactured by arranging onto a substrate a matrix of sub-pixels (organic EL elements) of three different colors: red (R), green (G), and blue (B). Each set of R, G, and B organic EL elements constitutes one pixel. The respective organic EL elements are manufactured by arranging, in order, pixel electrodes (e.g., anodes), organic emitting layers, and counter electrodes (e.g., cathodes) onto a substrate. In some cases, a functional layer including an electron injection layer, an electron transport layer, a hole transport layer, a hole injection layer, and the like is also formed.
The organic EL elements are typically of three types: organic EL element R which emits red light; organic EL element G which emits green light; and organic. EL element B which emits blue light. In some cases, as all of the organic emitting layers, white light-emitting luminescent layers may be made while providing color filters that change white light to desired colored light. In other cases, an organic red light-, green light-, or blue light-emitting layer is provided for each of the organic EL elements.
The functional layer including an organic emitting layer, a hole injection layer, and a hole transport layer is formed for instance by applying a coating solution of functional layer materials onto a substrate and drying the coating solution. More specifically, banks made of resin or the like are formed over the surface of the substrate to define spaces for each of R, G and B, where such a functional layer is to be provided. Subsequently, the coating solution is applied in the corresponding spaces defined by the banks and dried to form the functional layer.
When forming the functional layer by the coating method in this way, it is possible that the drying rate of coating solution differs between the center coating region (a region where a functional layers is to be formed) and the surrounding edge coating region of the panel. Variations in drying rates result in variations in thicknesses of functional layers to be formed. The variations in functional layer thickness among pixels lead to luminance variations across the display.
In an effort to overcome this problem, technologies have been proposed in which the coating region (a region where the functional layer is to be formed) at the edge of the panel (edge coating region) is made larger than the coating region at the center of the panel (center coating region) (see e.g., Patent Literature 1). In Patent Literature 1, the edge coating region is made larger than the center coating region, allowing the larger coating region to hold more coating solution than the smaller one. In this way, the difference in drying rate of coating solution between the center and edge coating regions of the panel is corrected.
Other proposed technologies involve setting the amount of solvent contained in the coating solution to be applied in the edge coating region of the panel larger than the amount of solvent contained in the coating solution to be applied on the center coating region, so that the difference in drying rate of the coating solution between the center and edge coating regions is corrected (see, e.g., Patent Literature 2). According to Patent Literature 2, although the volume of solvent of coating solution to be applied in each coating region varies, the organic material contained in the coating solution to be applied in each coating region is constant.
Yet another proposed technologies involve providing a pixel electrode-free region (dummy region) that surrounds a luminescent region consisting of a matrix of pixels, so that luminance variation across the display that occurs due to the difference in drying rate between the center region and surrounding edge region of the panel can be avoided (see, e.g., Patent Literatures 3 to 6).
Providing such a dummy region around the perimeter of the luminescent region and applying a coating solution in the dummy region in this way results in the formation of functional layers of varying thickness in the dummy region, which is not a luminescent region. However, the dummy region can reduce variations in drying rates of coating solutions across the luminescent region positioned at the center of the panel, so that the functional layers to be formed in the luminescent region are uniform in thickness among pixels. In this way, it is possible to reduce luminance variations across the display.
Moreover, as a technology for enhancing light extraction efficiency from organic EL elements, it has been reported to employ a transparent electrode for either of the pixel electrode and counter electrode; employ a reflective electrode for the other while disposing a transparent conductive film by sputtering or the like between the organic emitting layer and reflective electrode (see, e.g., Patent Literature 7). By appropriately adjusting the optical distance between the organic emitting layer and reflective electrode by means of the transparent conductive film disposed between them, the light beam reflected by the reflective electrode and then travels toward the transparent electrode and the light beam that directly travels toward the transparent electrode are combined together to increase light extraction efficiency.